Significant developments in communications have been made by the introduction of technologies that increase system operating efficiency (i.e., system “throughput”). One example of these technologies is the use of two or more transmit antennas and two or more receive antennas (i.e., multiple antennas) in a wireless communications system that employs multiple frequencies (i.e., multiple carriers). Such systems are typically referred to as Multi-Input, Multi-Output (MIMO) communications systems. In contrast, traditional wireless communications systems typically employ one transmit antenna and one receive antenna operating at a single signal-carrier frequency (SC), and such systems are referred to accordingly as Single-Input, Single-Output (SISO) systems.
In the operation of MIMO communications systems, signals are typically transmitted over a common path (i.e., a channel) by multiple antennas. The signals are typically pre-processed to avoid interference from other signals in the common channel. There are several techniques that may be used to pre-process the signals in this regard, and some of these techniques may be combined to further improve system throughput. One such technique, known as Space-Time Processing (STP), processes and combines “preambles” and “data symbols” into “space-time signal structures.” Wireless communications systems typically transmit data or information (e.g., voice, video, audio, text, etc.) as formatted signals, known as data symbols (or information symbols), which are typically organized into groups, known as data frames (or information frames).
Training symbols (or preamble symbols) are another type of symbol, which are typically added as prefixes to data symbols (e.g., at the beginning of data frames), to enable training (i.e., synchronization) of the data symbols between the transmitters and receivers of a MIMO communications system. These training symbol prefixes can be referred to as preambles or preamble structures. The combination of the preambles and data symbols can be referred to as space-time signal structures. Space-time structures may also be constructed using STP for preambles and data symbols individually. Furthermore, pilot structures (or pilots) are space-time structures that are also constructed by STP and have the same structure as preambles, although they are periodically arranged within groups of data symbols for different purposes. Certain properties incorporated into space-time signal structures make it possible to recover the data symbols from them through post-processing by a receiver, for example. Moreover, the formation and processing of space-time signal structures in a wireless communications system may provide increased strength (i.e., gain) in the recovered signal, which typically enhances the performance of the communications system.
Another technique that may be used to pre-process signals in MIMO communications systems is called Frequency Division Multiplexing (FDM). FDM involves dividing the frequency spectrum of a wireless communications system into sub-channels and transmitting modulated data or information (i.e., formatted signals for voice, video, audio, text, etc.) over these sub-channels at multiple signal-carrier frequencies (“sub-carrier frequencies”). Orthogonal Frequency Division Multiplexing (OFDM) has emerged as a popular form of FDM in which the sub-carrier frequencies are spaced apart by precise frequency differences. The application of OFDM technologies in SISO communications systems (i.e., SISO OFDM systems) provides the capability, among others, to transmit and receive relatively large amounts of information. The application of OFDM in MIMO communications systems (i.e., MIMO OFDM systems) provides the added capability of increased capacity to transmit and receive information using, generally, the same amount of bandwidth (i.e., transmission line capacity) as used in SISO OFDM systems. MIMO OFDM communications systems also offer improved performance to overcome some of the difficulties experienced in other FDM communications systems, such as performance degradation due to multiple versions of a transmitted signal being received over various transmission paths (i.e., multi-path channel interference).
In wireless communications systems (e.g., SISO or MIMO), synchronization of data symbols is typically required in both time and frequency. Estimation of noise variance and channel parameters is also typically required. Thus, efficient preamble structures and pilot structures for use in wireless communications systems should provide both synchronization and parameter estimation. Furthermore, efficient preamble structures and pilot structures should possess a low peak-to-average power ratio (PAPR) (i.e., at or approaching unity) to facilitate efficient system operation. In their application to MIMO communications systems, however, existing preamble structures and pilot structures have shortcomings in their capability to provide the foregoing functions of time and frequency synchronization, estimation of noise variance and channel parameters, and low PAPR. For example, the IEEE Standard 802.11a preamble structure includes a short sequence, which provides time synchronization and coarse frequency offset estimation, followed by a long sequence, which provides fine frequency and channel estimation. Although this preamble has direct application to SISO communications systems, it is not directly applicable to MIMO communications systems to provide the above mentioned functions, without the need for significant modifications.
Existing techniques for space-time processing of preamble symbols, pilot symbols, and data symbols into space-time structures also have shortcomings in their applications to MIMO communications systems. For example, existing space-time structures (i.e., preamble, pilot, or data) are typically limited to applications in MIMO communications systems that employ two, four, or eight transmit antennas. However, MIMO communications systems may be required that employ other numbers of transmit antennas to satisfy various applications. As another example, existing space-time structures do not support the “full diversity” performance of MIMO communications systems. That is, existing space-time structures do not support the optimal signal transmission performance that MIMO communications systems can provide. For example, a MIMO communications system that employs four transmit antennas can provide a full diversity signal transmission performance of four space-time structures over four time periods. However, typical existing space-time structures are limited to support a signal transmission performance of no better than three space-time structures over four time periods in a four-antenna MIMO system.
Therefore, there is a need for apparatus and methods for providing efficient preamble structures and pilot structures that provide time and frequency synchronization, estimation of noise variance and channel parameters, and low PAPR in their application to MIMO communications systems. Moreover, there is a need for an apparatus and methods for providing space-time structures (i.e., preamble, pilot, or data) that can be applied to MIMO communications systems with any number of transmit and receive antennas and that facilitate full diversity performance of MIMO communications systems.